This invention relates generally to shock-absorbing devices. The invention is particularly directed to a novel shock-absorbing joint and shock-absorbing assembly that absorb shock forces by distortion of elastomeric material under torque. The elastomeric material absorbs rotational shear forces resulting from such torque and by elastomeric spring action returns each joint, and thus the whole shock-absorbing assembly, to an original unloaded position.
The present invention is believed to be of greatest value as a shock-absorber for a dock or marine platform. The shock-absorbing joint and assembly of the present invention is, however, not limited to use in the marine environment. The present invention can be used to reduce the impact force between any two objects with sudden shock forces one against the other.
Shock-absorbing devices, such as bumpers, shock cells, and other cushioning devices, have been used for some time to protect offshore production and drilling platforms and marine vessels in the vicinity of such platforms. Similar devices have been used on docks and berths to protect both the dock and the vessel approaching the dock from damage that might result from sudden and forceful contact between the two. Generally, shock-absorbing devices are used to cushion the impact of vessels striking docks, offshore platforms, and the like. In the absence of such shock-absorbing devices, the uncushioned full shock load of a vessel against the structure of a platform or dock may damage or weaken such structure.
One particular device for use as a dock shock-absorber between an approaching ship and the dock is the shock-absorber assembly described in U.S. Pat. No. 3,988,013 to Von Bose. This patent describes a device employing a hydraulic cylinder, the stroke of which is amplified by a system of levers. The principal disadvantages associated with the use of any shock-absorbing device employing a hydraulic cylinder as its principal operating structure arise from the number of moving mechanical parts involved. In the highly corrosive marine environment, the principle handicap of mechanical shock mitigation devices is that of susceptibility to corrosion. One attempted solution to this problem is the additional expense of providing specially bonded seals for complete environmental protection, such as that described in the Von Bose patent.
It has also been known in the prior art for some time to use rubber or other elastomeric material in compression, shear, or both, to absorb docking impact and similar impact forces between docks or platforms and floating vessels. Regardless of the particular configuration employed, the functioning of such devices using rubber or similar materials relies upon the transformation of the momentum of the ship into molecular distortion forces within the elastomeric material.
One particular type of device that has been used in shock-absorbing marine fenders can best be referred to as an axial shock load absorbing cell. This particular type of shock cell is disclosed in U.S. Pat. Nos. 4,098,211 and 4,109,474 to Files et al. and in U.S. Pat. No. 3,991,582 to Waldrop et al. In the typical axial load cell disclosed in each of these three patents, elastomeric material is bonded between two different diameter cylinders. Upon receiving a shock load, the smaller diameter cylinder is made to move longitudinally relative to the larger diameter cylinder so as to work the rubber or other elastomeric material in a shear stress.
Another embodiment for axial load cells is that disclosed in U.S Pat. No. 4 084,801 to Landers et al. Prior to the invention described in the Landers patent, the typical axial load cell was produced in a one-step molding process wherein the rubber or other elastomeric material was molded in place between the two different diameter cylinders. The Landers patent provided some insight into coping with problems arising from shrinkage of the molded elastomeric material and the concomitant breaking of bonds of the elastomeric material from the cylinders. This improvement was essentially accomplished by providing for a plurality of molded and bonded segments within the annular space between the two cylinders of a given load cell.
There remain, however, significant design problems with the axial load cells of the type described in the Landers patent. For example, there remains the problem of more stress on the bond between elastomeric material and a cylinder than on the elastomeric material itself within the typical axial load cell. This is due in part to the discontinuity during shear stress at the extremities of the body of elastomeric material within an axial load cell. Other more fundamental disadvantages also exist for the axial load cells. The typical axial load cell has a very narrow range of variable design characteristics. The typical axial load cell has a relatively short stroke and a high spring rate. In the operation of such an axial load cell, the cell must have a long enough stroke at an appropriate spring rate so that the energy (which is primarily a function of vessel displacement and approach velocity) is absorbed before the load limit of the structure is reached. In order to increase the energy absorbing capacity of an axial load cell without increasing the reaction loads requires that the stroke of the axial cell be extended (and thus the spring rate lowered). There are, however, limited means available for lowering the spring rate of an axial load cell. If the axial length of the elastomeric material is reduced, the annular space must eventually be decreased, thus causing the spring rate to increase. The choice of spring rates is therefore critically limited in the use of axial load cells.
The shock-absorbing joint of the present invention overcomes many of these fundamental disadvantages of the axial load cell described above. Basically, the shock-absorbing joint with rotating arms of the present invention provides for a much more energy efficient shock-absorbing device, i.e., a higher energy absorbing system using the same amount of rubber is realized.
The shock-absorbing joint of the present invention also provides much greater flexibility in design. The shock-absorbing joints can be incorporated into an overall shock-absorbing assembly with articulating arms that can be made to move in either a vertical or horizontal plane. This feature provides additional design flexibility by accommodating those circumstances where the approaching vessels have significant vertical factors in the forces they bring to the structure being protected. In addition, the properties of the shock-absorbing joint of the present invention allow much more freedom in choosing the effective load versus the deflection characteristics of a shock-absorbing assembly. The number and size of joints are determined by the total energy absorption requirements both with respect to the magnitude and direction of the expected shock forces.
Because the joint of the present invention provides for working the elastomeric material under torque, each shock-absorbing joint will have a characteristic energy absorption capacity and spring rate. The spring rate may be defined as the moment developed per degree of rotation. The joints each have pivoting members or arms which convert the linear motion of the vessel to rotational motion about the joint. The effective spring rate is inversely proportional to the arm length. The arm length may thus be chosen to give the desired force level at maximum stroke. The effective spring rate and maximum force can be decreased to any level required by merely increasing the arm length, subject only to physical limitations as to the size of the overall shock-absorbing assembly. Thus, by adjusting the length of the articulating arms, the distance over which the movement of the vessel is stopped is increased, but the torque in the rubber remains the same.
The present invention allows for greater energy absorbing capacity per unit rubber. With the type of shear experienced by the rubber in the joint of the present invention, i.e., under torque, applicants anticipate that the shear can be as high as 400% as compared to a maximum of 200% for the typical axial load cell. Further, because of the particular shear line realized when distorting the elastomeric material under torque, the problem with bond fatigue present in axial load cells is essentially eliminated. This elimination of bond fatigue is realized because, unlike axial load cells, the joints of the present invention have no exposed corners where shear forces cause the pulling of the elastomeric material away from the surface to which it is bonded.